The ABCs of Snowmobiling

Let us spell it out for you

The snowmobile industry is notorious for its use of acronyms. There are acronyms for everything from sleds-RMK-to sled parts-RAVE-to companies-SLP.Acronyms seem to be especially popular with the internal workings of sled engines-like DPM, VES and the like. In fact, we use acronyms so often that we sometimes forget what they actually stand for. We just know that, for the most part, whatever they stand for, these things work.And that's a good thing, because the alphabet that we want to focus on-variable exhaust systems-has been one of the best improvements to snowmobile motors made in the past few years. We're going to look at the Polaris Variable Exhaust System (VES), the Ski-Doo Rotax Adjustable Variable Exhaust (RAVE), Yamaha Power Valve System and Arctic Cat's unique water flow path change on its Firecats.

Polaris VESSeveral Polaris models are equipped with the Polaris patented Variable Exhaust System. This unique exhaust valve management system changes the effective exhaust port height in the cylinder to provide maximum horsepower at high rpm without sacrificing fuel economy and engine torque at low to midrange throttle settings.In order to understand the operation and function of the VES we must first consider the characteristics of a two-stroke engine. The height of the exhaust port in a two-stroke engine cylinder has an effect on the total power output of an engine, as well as the rpm at which the power occurs.Exhaust systems are "tuned" by design to match engine exhaust port configuration and desired power delivery characteristics. Engines with relatively high exhaust ports (and exhaust pipe to match) produce more horsepower at high rpm, but only at the expense of low to midrange fuel economy and torque. On the other hand, low port engines provide good fuel economy in the midrange and make their power at relatively lower rpm, but will not produce as much peak horsepower for a given displacement range. In general, an engine designed for a touring snowmobile will have a relatively low exhaust port compared to an engine of the same displacement range designed for racing or high performance.The main components of the VES are the exhaust valve, valve housing, bellows, piston, return spring and valve cover.A guillotine-style exhaust valve is connected to a moveable piston. This piston is attached to a flexible bellows, forming two chambers. The lower chamber is connected to the cylinder by a drilled passageway located just above the exhaust port. The upper chamber is vented to atmospheric pressure. A valve return spring is located in the upper chamber between the piston and valve cover.At idle or low speeds, the exhaust valve is held in the low port position by the return spring. When throttle is applied (and rpm begins to increase) rising cylinder pressure is applied to the under side of the bellows via the drilled passage. This forces the exhaust valve upward against spring pressure. The valve continues to move upward toward the high port position as cylinder pressure, horsepower and rpm increase.

VES MaintenanceDue to the simplicity of the VES design, maintenance is limited to periodic inspection and cleaning of system components. The VES should be disassembled, inspected and cleaned (remove carbon deposits) every 1,000-2,000 miles, depending on operating conditions.Source: Polaris 2002 Snowmobile Field Update

Ski-Doo RAVEFor a two-stroke cycle engine to have high power capacity at high crankshaft speeds, a high volumetric or breathing efficiency is required and the fresh charge losses must be minimized. This is achieved by opening the exhaust port early (ATDC) and utilizing the resonant effects of the tuned exhaust system to control fresh charge losses.When an engine of this design is run at a medium speed, efficiency falls off quickly. The relatively high exhaust port effectively shortens the useful power stroke and because the exhaust system is tuned for maximum power, there is a large increase of fresh charge losses.As a result, the torque decreases along with a dramatic increase of the specific fuel consumption. Higher torque along with lower fuel consumption can be obtained at lower engine speeds if the time the exhaust port is open is reduced (ATDC).Bombardier-Rotax has patented a remarkably simple system to automatically change the exhaust port height based on pressure in the exhaust system.Located above the exhaust port is a guillotine-type slide valve. This rectangular valve is connected by a shaft to a diaphragm which is working against the return spring. Two small passages just outside the cylinder in the exhaust port allow exhaust gas pressure to reach the diaphragm.As the throttle is opened and the engine begins producing more power, the pressure against the diaphragm by exhaust gas will overcome the pressure of the return spring and the RAVE valve will open.Over the return spring is a red plastic adjustment knob. Turning the adjustment in or out changes the preload on the return spring, which in turn will change the rpm at which the RAVE valve opens and closes. Depending on engine type, the exhaust port height changes a total of 4 to 6 mm when the RAVE valve is fully closed or fully opened.The RAVE valve does not allow the engine to make higher peak horsepower than an engine not so equipped; it can make moving the horsepower peak higher more practical because of its effect on the rest of the power curve.

RAVE AdjustmentThe red cap on the RAVE valve cover should be turned all the way in and bottomed in normal use. Backing the red adjuster out will reduce the spring preload and allow the RAVE valve to open at a lower rpm. Different return springs corresponding to wire diameter and length are used on various engine types.

RAVE MaintenanceThere is very little that wears in this system and there are no adjustments to be periodically checked. The only possible maintenance required would be cleaning of carbon deposits from the guillotine slide. Cleaning intervals would depend upon the user's riding style and the quality of the oil used. Even when using Ski-Doo injection oil, it is suggested that there be an annual cleaning of the valve. If a customer uses a lower quality, high ash oil, more frequent cleaning may be required.No special solvents or cleaners are used when cleaning the valve.Source: Ski-Doo 2002 Tech Manual

YPVSThe Yamaha Power Valve System improves engine performance at low and midrange rpm. The YPVS features computerized digital control units and the system is cable operated for precision performance. The self-powered system (no battery, powered by the stator) has two self cleaning cycles, which help to prevent carbon build-up on the valves.The first self-cleaning cycle happens when the rpm reaches 2500 after initial start up. The second cycle happens after the sled exceeds 6500 rpm and then drops below 3500 rpm.